1. Field of the Invention
This invention is in the field of forming uniform diameter nozzles in a tube of glass or other thermoplastic material utilizing stretching of the tube by means of a weight applied thereto as a portion of the tube is at a drawing temperature.
2. Description of the Prior Art
In two previous patents issued to me solely or jointly, U.S. Pat. Nos. 3,403,084 and 3,489,544, there are disclosed electrochemical machining processes using glass nozzles and also apparatus for making such nozzles. The known electrochemical machining processes involve penetration of a nozzle-type tool into a workpiece being machined as cavity sinking progresses, requiring not only very accurately sized internal nozzle tip diameters, but also very thin nozzle walls in order that the cavity formed in the workpiece be close to the diameter of the electrolyte stream forming the cavity and still be large enough to accept the tip end of the nozzle.
In U.S. Pat. No. 3,556,758 in which the present applicant is a co-inventor, there is described a method for drawing a hollow glass member which involves rotating the hollow glass member while applying a tube stretching force which is less than that which will produce plastic deformation at room temperature, applying to a selected portion of the tube heat at a heating rate sufficient to allow the tube stretching force to elongate the tube to a predetermined axial tube length, and thereafter heating the portion of the tube at a slower heating rate to control further elongation of the portion of the tube until the final axial tube length is achieved.
In any tube drawing process which involves suspending a weight at the bottom tube end and rotating the tube while it is being heated, it frequently occurs that the drawn tube does not have a true centerline, i.e., it may have a small bend which must be removed if the tube is to be used for precision purposes such as an electrode in an electrochemical machining process. The non-uniformity in the tube can be caused by several factors. For one, glass tubes as supplied frequently have slight bends in them and rotating such tubes while heating them accentuates the problem. Secondly, the gripping means which are employed in glass tube drawing machines are not always perfect in that they may cause rotation of the tube along other than the true vertical centerline in which case the drawn tube will again not have a uniform centerline.
With the use of tube drawing machines of the prior art, it is usually necessary to have personnel assigned to the job of doing nothing but straightening the tubes after they have been drawn. This procedure is costly and time consuming and adds a significant amount of cost to the final product.
The need still remains, therefore, for a tube drawing method and apparatus which will overcome the difficulties of the prior art and provide accurate drawing of a tube to a true centerline in all cases. The satisfaction of that need is the principal object of the present invention.